Fix check for missing goog.require directives

Take this example:
```
goog.require('ol.projection.addCommonProjections');

ol.projection.addCommonProjections();
```

Without this patch, the build_check_requires_timestamp target would report
that ``ol.projection`` is missing.
This is because ``ol.projection.addCommonProjections();`` would match both
``ol.projection.addCommonProjections and ``ol.projection`` provide
directives.
Merge pull request #370 from bbinet/fix-missing-requires

Fix check for missing goog.require directives